This invention relates generally to automatic transmissions mounted on motor vehicles and, more particularly, to an automatic transmission capable of being suitably mounted on a front engine front drive type small vehicle which requires a reduction in the size of the transmission. More specifically, the present invention relates to a controller for controlling back pressures in accumulators.
Conventionally, in an ordinary hydraulic controller for automatic transmissions, accumulators are provided in association with hydraulic servos for operating clutches and brakes, and pressure for engaging each of the clutches and the like is applied according to the accumulator characteristics in order to reduce shift shocks.
In each of particular accumulators, a back pressure chamber is formed at the rear of the piston by being defined with a stepped portion. Also, an accumulator valve controlled on the basis of a throttle pressure is provided to regulate a line pressure and to supply the regulated hydraulic pressure to the back pressure chambers of the accumulators to further reduce the intensity of a shift shock caused when the throttle opening is small.
Also, an arrangement, such as the one disclosed in Japanese Laid-Open Patent No. 61-130653, is known in which a special solenoid valve controlled on the basis of a signal from a throttle opening sensor may be provided to control an accumulator control valve for the same use.
In automatic transmission hydraulic controllers having the above construction, however, the accumulator control valve controlled on the basis of the throttle pressure regulates the line pressure and supplies the regulated pressure to the accumulator back pressure chambers, and it is therefore necessary to form the back pressure chambers with stepped portions having much smaller pressure receiving area than that of the accumulator chamber. It is also necessary to form the respective back pressure chambers defined by stepped portions into different shapes according to the engaging characteristics of the clutches or brakes.
In the case of the accumulator control valve controlled by means of a solenoid valve, the back pressure chambers are also supplied with a hydraulic pressure obtained by regulating a line pressure in the same manner, and it is necessary to form the back pressure chambers with stepped portions. Also, since the special solenoid valve is required for the accumulator control valve, the cost of the transmission is increased.
Where either accumulator control valve is used, the back pressures in the accumulators are controlled through a hydraulic pressure based on a line pressure, and back pressure chambers formed with stepped portions are therefore required. Accordingly, the length of the piston of each accumulator is increased. Also, it is necessary for each accumulator to have a specific shape according to the characteristics of the corresponding clutch or brake; it is difficult to determine a common shape for all the accumulators. These conditions make it difficult to reduce the size of each accumulator which occupies a substantially large space in the valve body and, hence, to mount the transmission on a small motor vehicle.